Attention is being given to a hierarchical storage system in which different types of storage having different characteristics are logically combined into one storage device. In the hierarchical storage system, fast but small-capacity and high-price storage devices are placed in higher rank and large-capacity and low-price but slow storage devices are placed in lower rank. For example, relatively high-price and high-speed storage devices such as semiconductor or magnetic disks are placed in higher rank and relatively low-price and low-speed storage devices such as optical disks and magnetic tape libraries are placed in lower rank.
Such hierarchical storage devices provide a transparent hierarchical structure in response to external file access. That is, as shown in FIG. 1, access to a hierarchical storage system 50 is executed via a file access point 10 that is present only in the first-level storage device 124; it is not taken into account during access which level of first-level storage device 124, second-level storage device 126, third-level storage device 128, or fourth-level storage device 129 an object file is present.
a. For example, a file stored in the first-level storage device 124 transfers to the second-level storage device 124 after two weeks and to the fourth-level storage device 129 after three years according to its attribute information (for example, the last access date). However, if an external file access request is given, the file is transferred or replicated among the levels back to the first-level storage device 124, and is finally accessed from the only file access point 10 in the first-level storage device 124. In this specification, “file access point” indicates information for searching for an object file in a hierarchical structure: examples for a network are MAC address and IP address, and for a file system, a path. Although actual hierarchical storage systems such as storage system 50 have a controller for controlling the input and output to/from the first-level storage device 124, second-level storage device 126, third-level storage device 128, and fourth-level storage device 129, FIG. 1 omits the controller for clarity.
In a conventional hierarchical storage systems, such as storage system 50, the file access point, such as file access point 10 is present only in the first-level storage device 124; file transfer or replication among first-level storage device 124, second-level storage device 126, third-level storage device 128, and fourth-level storage device 129 is executed from one storage device to the storage device directly above it or from one storage device to the storage device directly below it in sequence without skipping the intermediate levels (refer to FIG. 1). Therefore, if file transfer processing or file replication processing is executed in one of first-level storage device 124, second-level storage device 126, third-level storage device 128, or fourth-level storage device 129 and when an object file is present in a plurality of storage devices of different levels such as first-level storage device 124, second-level storage device 126, third-level storage device 128, and fourth-level storage device 129, a storage device at a level that is logically closest to a destination storage device is unconditionally selected as a source storage device.
Accordingly, the data transfer or replication among first-level storage device 124, second-level storage device 126, third-level storage device 128, and fourth-level storage device 129 in the conventional hierarchical storage systems 50 has the problem of wasting the resources or the first-level storage device 124, second-level storage device 126, third-level storage device 128, or fourth-level storage device 129 present between the source storage device and the destination storage device and energy for operating the resources. The data transfer or replication among storage devices first-level storage device 124, second-level storage device 126, third-level storage device 128, and fourth-level storage device 129 in the conventional hierarchical storage systems 50 further has the problem of requiring much time.